Sift flour, salt, bicarbonate of soda, cream of tartar together. Rub in butter until mixture looks like bread crumbs. Dissolve sugar in milk, stir into flour/butter mixture. Form into stiff dough. Roll out to a thickness of 1/4 inch. Cut in squares; prick all over. Bake in oven at 350 F (180 C) for 20 minutes.